1. Field of the Invention
This invention relates generally to variable displacement engines for providing improved fuel economy.
2. Background of the Invention
One method for selecting a number of cylinders to carry out combustion is described in U.S. Pat. No. 5,479,898. In this system, engine output is reduced via ignition timing, air-fuel ratio, and cylinder deactivation in a systematic approach. However, the inventors herein have recognized a disadvantage with such a system when using a new system configuration.
In particular, the inventors herein have recognized that operating with some cylinders combusting a lean air-fuel mixture and others inducting air without injected fuel can provide significantly increased fuel economy. Further, if an emission control device (that retains NOx during lean operation and later releases and reduces the NOx during rich operation) is coupled to the combusting cylinders, then such operation can be achieved without degraded emissions.
However, the inventors herein have also recognized that when exhaust gas recirculation, or EGR, is added to such operation, degraded airflow control can result.
By operating according to claim 1, it is possible to provide a requested airflow, and air to fuel ratio, even in the presence of EGR. i.e., since the EGR will displace fresh charge, and thereby change the combustion air-fuel ratio (since the recirculated exhaust gas includes a lean burnt mixture as well as fresh charge from the cylinders without fuel injection), the necessary airflow to meet a requested combustion air-fuel ratio is affected by the EGR.